1. Field of the Invention
The present invention relates to a camera exposure control system, and particularly to a camera exposure control system which measures the brightness of arbitrary portions of the photographic view field a plurality of number of times and determines the exposure value based on the measured values.
2. Description of the Prior Art
There have been known automatic exposure control systems of cameras which select a plurality of portions in the photographic view field, such as the principal object, background section, etc., which are considered to be crucial for the exposure control, measure the brightness of these portions, calculate a value (e.g., mean value) from the measured light values, and implement the exposure control based on the calculated value. Among such automatic exposure control systems, some are designed to switch the normal photography mode based on the multi-spot light values to the flash photography mode when a slower shutter speed (longer shutter opening time) is expected due to a darker photographic object or the like.
In the above-mentioned automatic exposure control system, once control is switched to the flash photography mode, the exposure control specific to flash light photography takes the place of the exposure control based on multi-spot light values. Namely, in a flashmatic control system, a value of aperture stop is determined from given values of the guide number indicative of the light output of the flash lamp and the sensitivity of the film and from the distance to the object, but without using measured light values of the photographic object. In an automatic flash light adjustment control system, a flash lamp is activated with the aperture stop being set arbitrarily, and is disactivated the flash lamp when the intensity of reflected light from the object has reached the proper exposure value, and it does not use measured light values of the object.
Accordingly, in order for the automatic exposure control system to use the function of exposure control based on the multi-spot light measurement effectively, it needs to disable the flash photography mode. However, no camera has such operating mode, and therefore the multi-spot light measurement to have a proper exposure is carried out in vain due to automatic switching to the flash photography mode. Some flash device attached externally to a camera is not designed to receive a flash inhibit signal from the camera, and also in this case the multi-spot light measurement to have a proper exposure is carried out in vain due to automatic switching to the flash photography mode.
There has been also known an exposure control system having a program mode for determining the exposure control values to be set on the camera, i.e., values of aperture stop and shutter speed, in which combinations of aperture stop values and shutter speeds are determined in advance for exposure values which are determined from light values of objects and film sensitivities, and a set of aperture stop and shutter speed is selected based on the determination of the exposure value in response to the measured light value and the film sensitivity signal. Also known are program shift means which, if the determined aperture stop and shutter speed are not desirable, alter the aperture stop or shutter speed to the intended value while retaining the exposure value. Some is designed to memorize the shift value so that it implements the program shift automatically by that shift value in the later photographic operation.
The exposure control system of this type is designed such that when the brightness of arbitrary portions of the photographic view field is measured with a multi-spot light measuring device, a newly measured light value at each reading is rendered a computation with older values which have already been read and the mean value, for example, of the measured light values is produced as the brightness of the photographic view field to be used for the exposure control. The exposure control in the program mode results in the variation of the exposure value and accordingly in the variation of the aperture stop and shutter speed at each reading of the light value.
On this account, once the photographer has used the program shift and thereafter the measured light value is read, a proper exposure is not achieved by the values of aperture stop and shutter speed based on the program shift. It requires another setting of the aperture stop and shutter speed based on a new exposure value, but the conventional exposure control system does not cope with this situation.